This invention relates to a continuous wave ranging system and, in one aspect, to an aircraft radar altimeter system.
Such systems usually comprise a means of microwave transmission upon which some form of coding has been added, and antenna for directing the energy to the target, an antenna for receiving the returned energy and, after amplification, a means of determining the amount of delay that has occurred on the signal, and hence the range of the target. The coding on the transmission had in the past been pulse or frequency modulation, but more recently phase modulation from a pseudo-random code has been used. This form of modulation has the property of producing a noise-like transmitted spectrum which is difficult to detect and hence finds applications where covertness is of importance. Covertness can be enhanced by reducing the transmitted power such that the returned signal is just sufficient for ranging measurement.
In such phase-modulated systems, the received signal is correlated with a delayed version of the transmitted code, the delay being gradually increased in steps, and samples of the output of the correlator are detected and stored in an array. From this stored data, the delay, and hence the range, where the received signal return occurs, can be found.
Existing direct sequence spread spectrum ranging systems use techniques such as delay locked or Tau dither loops to track target ranges. These techniques result in a narrow tracking window and tracking loops with excellent dynamic performance. However, the narrowness of the tracking window restricts the ability of such systems to see any targets at ranges other than that being tracked.